Systems and methods for operating a furnace

ABSTRACT

Methods and related systems for operating a furnace are disclosed. In an embodiment, the method includes activating a burner assembly and a first fan of the furnace to combust fuel and air and circulate combustion gases along a flow path extending through a heat exchanger of the furnace. In addition, the method includes operating a second fan of the furnace to circulate air across an external surface of the heat exchanger of the furnace and produce a conditioned airflow. Further, the method includes monitoring one or more parameters of a motor of the second fan indicative of an airflow rate of the conditioned airflow, and deactivating the burner assembly, whereby combustion of the fuel and air in the furnace ceases, in response to the one or more parameters indicating that the airflow rate is less than a minimum airflow rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

Climate control systems, such as heating, ventilation, and/or airconditioning (HVAC) systems may generally be used in residential and/orcommercial areas for heating and/or cooling to create comfortabletemperatures inside those areas. Climate control systems may include agas-fired furnace generally configured to combust fuel and air andthereby generate heat, which may be delivered to a comfort zone. Thefurnace may include a burner assembly generally configured to combustfuel and air whereby resultant combustion gases are forced into andthrough a heat exchanger of the furnace. The furnace may also include acirculation fan or blower to produce an airflow across an externalsurface of the heat exchanger to transfer thermal energy from the heatexchanger to the airflow, which may be delivered to the comfort zone.

BRIEF SUMMARY

Some embodiments disclosed herein are directed to a method for operatinga furnace. In an embodiment, the method includes activating a burnerassembly and a first fan of the furnace to combust fuel and air andcirculate combustion gases along a flow path extending through a heatexchanger of the furnace. In addition, the method includes operating asecond fan of the furnace to circulate air across an external surface ofthe heat exchanger of the furnace and produce a conditioned airflow.Further, the method includes monitoring one or more parameters of amotor of the second fan indicative of an airflow rate of the conditionedairflow, and deactivating the burner assembly, whereby combustion of thefuel and air in the furnace ceases, in response to the one or moreparameters indicating that the airflow rate is less than a minimumairflow rate.

Other embodiments disclosed herein are directed towards a furnace. In anembodiment, the furnace includes a burner assembly configured to combustfuel and air to produce combustion gases, a first fan configured tocirculate the combustion gases along a flow path extending through aheat exchanger of the furnace, and a second fan configured to circulateair across an external surface of the heat exchanger to produce aconditioned airflow. In addition, the furnace includes a controllerconfigured to activate the burner assembly and the first fan to combustthe fuel and air and circulate the combustion gases along the flow path,and operate the second fan to circulate the air across the externalsurface of the heat exchanger and produce the conditioned airflow. Inaddition, the controller is configured to monitor one or more parametersof a motor of the second fan indicative of an airflow rate of theconditioned airflow, and deactivate the burner assembly wherebycombustion of the fuel and air ceases in response to the one or moreparameters indicating that the airflow rate is less than a minimumairflow rate.

Further embodiments disclosed herein are directed to a non-transitorymachine-readable medium. In an embodiment, the non-transitorymachine-readable medium includes instructions that, when executed by aprocessor, cause the processor to activate a burner assembly and a firstfan of a furnace to combust fuel and air and circulate combustion gasesalong a flow path extending through a heat exchanger of the furnace, andoperate a second fan of the furnace to circulate air across an externalsurface of the heat exchanger of the furnace and produce a conditionedairflow. In addition, the instructions, when executed by a processor,cause the processor to monitor one or more parameters of a motor of thesecond fan indicative of an airflow rate of the conditioned airflow, anddeactivate the burner assembly, whereby combustion of the fuel and airceases, in response to the one or more parameters indicating that theairflow rate is less than a minimum airflow rate.

Embodiments described herein comprise a combination of features andcharacteristics intended to address various shortcomings associated withcertain prior devices, systems, and methods. The foregoing has outlinedrather broadly the features and technical characteristics of thedisclosed embodiments in order that the detailed description thatfollows may be better understood. The various characteristics andfeatures described above, as well as others, will be readily apparent tothose skilled in the art upon reading the following detaileddescription, and by referring to the accompanying drawings. It should beappreciated that the conception and the specific embodiments disclosedmay be readily utilized as a basis for modifying or designing otherstructures for carrying out the same purposes as the disclosedembodiments. It should also be realized that such equivalentconstructions do not depart from the spirit and scope of the principlesdisclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various exemplary embodiments, referencewill now be made to the accompanying drawings in which:

FIG. 1 is a perspective exploded view of a furnace according to someembodiments;

FIG. 2 is an orthogonal view of the furnace of FIG. 1 ;

FIG. 3 is a conditioned airflow temperature map of a gas-fired furnaceaccording to some embodiments;

FIG. 4 is a flow chart of a method for operating a furnace according tosome embodiments; and

FIG. 5 is a flow chart of another method for operating a furnaceaccording to some embodiments.

DETAILED DESCRIPTION

The following discussion is directed to various exemplary embodiments.However, one of ordinary skill in the art will understand that theexamples disclosed herein have broad application, and that thediscussion of any embodiment is meant only to be exemplary of thatembodiment, and not intended to suggest that the scope of thedisclosure, including the claims, is limited to that embodiment.

The drawing figures are not necessarily to scale. Certain features andcomponents herein may be shown exaggerated in scale or in somewhatschematic form and some details of conventional elements may not beshown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .” Also, theterm “couple” or “couples” is intended to mean either an indirect or adirect connection. Thus, if a first device couples to a second device,that connection may be through a direct connection of the two devices,or through an indirect connection that is established via other devices,components, nodes, and connections. In addition, as used herein, theterms “axial” and “axially” generally mean along or parallel to a givenaxis (e.g., central axis of a body or a port), while the terms “radial”and “radially” generally mean perpendicular to the given axis. Forinstance, an axial distance refers to a distance measured along orparallel to the axis, and a radial distance means a distance measuredperpendicular to the axis. Further, when used herein (including in theclaims), the words “about,” “generally,” “substantially,”“approximately,” and the like mean within a range of plus or minus 10%.

As described above, a gas-fired furnace is generally configured tocombust fuel and air and thereby generate heat that may be delivered toa comfort zone of an indoor area via a circulation fan or blower of thefurnace. Particularly, the furnace may be configured to provide atemperature rise between a temperature of an inlet airflow of thefurnace and a temperature of a conditioned airflow of the furnace. Thetemperature rise is targeted to a predefined temperature rise range inwhich the furnace is designed to operate. Thus, the temperature riserange of the furnace may depend upon the configuration of the furnace.For instance, a first exemplary furnace having a first configuration mayhave a temperature rise range between 30 degrees Fahrenheit (° F.) and60° F., while a second exemplary furnace, having a second configurationthat is different from the first configuration, may have a temperaturerise range between 40° F. and 70° F.

The temperature rise produced by the furnace may vary during operationin response to changes in the amount of airflow through the furnaceproduced by the circulation fan. Generally, at a given rate ofcombustion within the furnace, the temperature rise provided by thefurnace between a temperature of the inlet airflow and a temperature ofthe conditioned airflow may increase in response to a decrease in theamount of airflow circulated through the furnace. In other words, adecrease in the airflow rate provided by the circulation fan results inless volume of air available to absorb the heat generated within thefurnace over a given period of time. Conversely, the temperature riseprovided by the furnace may decrease in response to an increase in theamount of airflow circulated through the furnace (i.e., an increase inthe airflow rate provided by the circulation fan) as more airflow isavailable to absorb the heat generated within the furnace over a givenperiod of time.

In some applications, if the temperature rise provided by the furnacedecreases below a lower end of the temperature rise range of thefurnace, the temperature of the conditioned airflow may not be warmenough to adequately heat the comfort zone. Additionally, if thetemperature rise provided by the furnace increases above the temperaturerise range of the furnace, the temperature of the conditioned airflowmay exceed a maximum permissible conditioned airflow temperature of thefurnace. The maximum permissible conditioned airflow temperature of thefurnace may vary depending upon the application and the configuration ofthe furnace, but in some applications may correspond to a temperatureequal to an upper end of the temperature rise range of the furnace plusa fixed margin which may, in some applications, range approximatelybetween 100 degrees Fahrenheit (° F.) and 200° F. As an example, themaximum permissible conditioned airflow temperature for a furnace havinga temperature rise range of 30° F. and 60° F. would be 160° F. when thefixed margin is equal to 100° F. Additionally, a conditioned airflowtemperature exceeding the maximum permissible conditioned airflowtemperature of the furnace may eventually result in damage to thefurnace due to overheating of the furnace and/or heat-related discomfortto occupants of the comfort zone of the indoor area. A conditionedairflow temperature produced by the furnace that is in excess of atarget conditioned airflow temperature (e.g., as requested by acontroller of the furnace) may result from improper installation of thefurnace, a malfunction of the circulation fan or other component of thefurnace, and/or a clogged filter or air cleaner of the furnace.

In conventional furnaces, the furnace includes a temperature limitswitch, such as a spring-operated bimetallic switch. The switch opensautomatically to shut off the burner in response to a temperature of thebimetallic switch reaching a predefined set-point temperature. Using abimetallic switch to trigger shutoff has drawbacks. For example, theperformance of the bimetallic switch may be orientation specific,requiring the switch to be installed within the furnace in a particularorientation for the switch to function as intended, limiting theflexibility in which the components of the furnace may be internallypositioned, as well as the ways in which the furnace may be oriented inthe indoor space. Additionally, a bimetallic switch is a separatecomponent that adds cost to the furnace.

Accordingly, embodiments disclosed herein include systems and methodsfor operating a furnace whereby a temperature limit switch, such as aspring-operated bimetallic switch, is rendered superfluous in preventingthe furnace from exceeding its designed temperature rise range.Particularly, embodiments disclosed herein include systems and methodsfor operating a furnace that includes operating a gas-fired furnace toproduce a conditioned airflow, monitoring one or more parametersindicative of an airflow rate of the conditioned airflow, anddeactivating a burner assembly of the furnace in response to the one ormore parameters indicating that the airflow rate is less than a minimumairflow rate. Embodiments disclosed herein also include systems andmethods for operating a furnace that includes operating a gas-firedfurnace to produce a conditioned airflow, monitoring a parameter that isindirectly indicative of a temperature of the conditioned airflow, anddeactivating a burner assembly of the furnace in response to theparameter indirectly indicating that the temperature of the conditionedairflow exceeds a threshold. As will be described in more detail below,use of the embodiments disclosed herein may allow for the operation of afurnace in a manner that prevents the furnace from exceeding a maximumpermissible conditioned airflow temperature thereof without needing torely on a temperature limit switch, such as a spring-operated bimetallicswitch.

Referring now to FIGS. 1, 2 , an embodiment of a gas-fired furnace 100is shown. As discussed herein, a furnace (e.g., furnace 100) may bereferred to as being “gas-fired”, where the “gas-fired” furnace isconfigured to be in fluid communication with a gas flow forthermodynamic heat transfer and where the gas-flow comprises products ofa combustion reaction from a burner. In some embodiments, furnace 100may comprise a component of an HVAC system that includes an indoor unitcomprising furnace 100 and an indoor refrigerant heat exchanger orevaporator, an outdoor unit comprising an outdoor fan and an outdoorrefrigerant heat exchanger or condenser, and a refrigerant loopextending between the indoor and outdoor refrigerant heat exchangers.Furnace 100 may configured as an indoor furnace that providesconditioned air to a comfort zone of an indoor space. However, ingeneral, the components of furnace 100 may be equally employed in anoutdoor or weatherized furnace to condition an interior space. Moreover,furnace 100 may be used in residential or commercial applications.

In some embodiments, furnace 100 may generally include a fuel supplyvalve 102, an air and fuel (air/fuel) mixing unit 110, an intakemanifold 120, a partition panel 130, a burner assembly 140, a pluralityof heat exchangers 150, a hot collector box 160, and a first fan ordraft inducer 170. Mixing unit 110 may be coupled end-to-end with intakemanifold 120. Additionally, burner assembly 140 may be positionedbetween intake manifold 120 and heat exchangers 150, where heatexchangers 150 may extend from burner assembly 140 to hot collector box160.

The air/fuel mixing unit 110 of furnace 100 may be configured for theintroduction of fuel and air to allow at least partial mixing of fueland air before a combustion reaction process. Air/fuel mixing unit 110may receive air via an air inlet 112 and fuel via fuel supply valve 102to allow at least partial mixing of the fuel and air. For example, thefuel may be natural gas available from the fuel supply valve 102attached and operatively engaged with the air/fuel mixing unit 110. Fuelsupply valve 102 may be configured to be adjusted, such as electricallyor pneumatically, so as to obtain a desired and/or predefinedair-to-fuel ratio. As will be discussed further herein, fuel supplyvalve 102 may be configured for staged operation and/or modulation typeoperation, and may be operatively connected to a controller 190 (shownschematically in FIG. 2 ) of furnace 100. For example, staged operationmay comprise two flame settings, whereas modulation type operation maybe incrementally adjustable over a large range of outputs, such as, forexample, from 40% to 100% output capacity. While furnace 100 is shown inFIGS. 1, 2 as comprising a premix furnace configured to mix air and fuelwithin air/fuel mixing unit 110, in other embodiments, furnace 100 maynot include air/fuel mixing unit 110 and may instead be configured tomix the fuel and air within burner assembly 140.

The intake manifold 120 of heat exchanger 100 may generally include aflow distributor 122 extending from an inlet of intake manifold 120coupled with air/fuel mixing unit 110. Intake manifold 120 may alsoinclude a plurality of heat exchanger supply tubes 124 extending fromflow distributor 122 to an outlet of intake manifold 120 coupled withheat exchanger 150.

The burner assembly 140 of furnace 100 may include a plurality ofburners 142 and at least one igniter 144 (shown in FIG. 2 ). Each burner142 of burner assembly 140 may be received in one of the supply tubes122 of intake manifold 120. Igniter 144 of burner assembly 140 may bepositioned at an opening of each burner 142 and may be configured toinduce a combustion reaction by igniting a gas flow passing in and/or byburners 142, where the gas flow comprises a mixture of the air and fuel.Particularly, the gas flow may initially take the form of air and fuelthat is at least partially mixed and/or uncombusted (i.e., not yetignited or undergone a combustion reaction) in air/fuel mixing unit 110.As the gas flow travels through intake manifold 120 and heat exchanger150, burners 142 and igniter 144 of burner assembly 140 may initiate acombustion reaction. Combustion may occur at least partially within aninterior space of each burner 142 so that heat is generated and forcedout of the open end of the burner 142 and into the heat exchanger tube150. In some embodiments, igniter 144 may comprise any of a pilot light,a piezoelectric device, and/or a hot surface igniter. Igniter 144 may becontrolled by controller 190 of furnace 100.

In some embodiments, the heat exchanger 150 of furnace 100 has a firstend 152 coupled to intake manifold 120 and a second end 154 coupled tohot collector box 160. Heat exchanger 150 may comprise an exteriorsurface 156 and a plurality of heat exchanger tubes 158 extendingbetween the first end 152 and the second end 154. In some embodiments,each heat exchanger tube 158 is a bent, S-shaped tube that extendsthrough a tortuous path to enhance the surface area available for heattransfer with the surrounding circulation air. However, in otherembodiments, the configuration of heat exchanger 150 may vary. In someembodiments, a finned condensing heat exchanger 165 may extend from hotcollector box 160 to draft inducer 170. However, generally, furnace 100may be operated with or without a condensing heat exchanger as a“condensing” or “non-condensing” furnace, respectively.

In some embodiments, the gas flow may follow a combustion flow path(indicated by arrow 172) that may be in a direction beginning at theair/fuel mixing unit 110 and ending at the draft inducer unit 170. Forexample, combustion flow path 172 may follow from the air/fuel mixingunit 110, through intake manifold 120, past burners 162 and through heatexchanger tubes 158 of heat exchanger 150. Combustion flow path 172 maycontinue through hot collector box 160 and condensing heat exchanger165, and may exit past draft inducer 170 towards a designated ventingenvironment (not shown in FIGS. 1, 2 ). It is understood that there maybe more or less components of furnace 100 in fluid communication withcombustion flow path 172.

In some embodiments, the gas flow described above may be introduced intofurnace 100 by operating in an induced draft mode by pulling the gasflow through furnace 100 via draft inducer 170, or by operating in aforced draft mode by pushing the gas flow through furnace 100. Draftinducer 170 may comprise a blower or fan which is in fluid communicationwith combustion flow path 172 and is down-stream of heat exchanger 150.Draft inducer 170 may pull and/or extract the gas flow out from heatexchanger 150 by creating a relatively lower pressure at one end ofcombustion flow path 172. Embodiments using a forced draft mode may beaccomplished by placing a blower or fan at the inlet of air/fuel mixingunit 110 and forcing the gas flow into and through air/fuel mixing unit110 and along combustion flow path 172.

As shown particularly in FIG. 2 , in addition to partition panel 130,furnace 100 may also include a first side panel 132, and a second sidepanel 134. Panels 130, 132, and 134 may be disposed in a configurationsuch that fluids (e.g. air) that contact an exterior surface of acomponent of furnace 100 (e.g. fluid passing over the exterior surface156 of heat exchanger 150 for thermodynamic heat transfer) aresegregated from the gas flow circulating along combustion flow path 172.

Furnace 100 may further include a second or circulation fan 180.Circulation fan 180 may be configured to receive an inlet airflow 182and force or drive the inlet airflow 182 into contact with the exteriorsurface 156 of heat exchanger 150. In other embodiments, circulation fan180 may draw the airflow 182 across the exterior surface 156 of heatexchanger 150. In response to the inlet airflow 182 contacting heatexchanger 150, heat may be transferred from the gas flow circulatingwithin heat exchanger 150 to the inlet airflow 182, thereby heatinginlet airflow 182. Following contact with heat exchanger 150, theairflow may exit furnace 100 as an outlet or conditioned airflow 184,which may have a temperature that is greater than a temperature of theinlet airflow 182. Conditioned airflow 184 may be delivered to a comfortzone of an indoor space.

In some embodiments, circulation fan 180 may comprise a centrifugalblower comprising a blower housing 181, and a blower motor 183configured to selectively rotate a blower impeller 185 of thecirculation fan 180 that is at least partially disposed within blowerhousing 181. In other embodiments, circulation fan 180 may comprise amixed-flow fan and/or any other suitable type of fan. Circulation fan180 may be configured as a modulating and/or variable speed fan capableof being operated at many speeds over one or more ranges of speeds. Inother embodiments, circulation fan 180 may be configured as a multiplespeed fan capable of being operated at a plurality of operating speedsby selectively electrically powering different ones of multipleelectromagnetic windings of the motor 183 of circulation fan 180.

As shown particularly in FIG. 2 , furnace 100 may comprise controller190 for controlling one or more components of furnace 100. Generallyspeaking, controller 190 is coupled to various components of furnace 100as well as various sensors configured to detect various operatingparameters within furnace 100. For example, in some embodiments,controller 190 of furnace 100 may communicate with and/or otherwiseaffect control over fuel supply valve 102, igniter 144 of burnerassembly 140, draft inducer 170, and/or circulation fan 180.Additionally, controller 190 may control draft inducer 170 to provide anadequate gas flow along combustion flow path 172 for a desired firingrate through burner assembly 140.

Controller 190 may comprise a singular controller or control board ormay comprise a plurality of controllers or control boards that arecoupled to one another. For example, controller 190 may comprise adistinct control board positioned on a panel (e.g., panels 130, 132,and/or 134) of furnace 100 and/or a control board positioned within themotor 183 of circulation fan 180. For convenience, and to simplify thedrawings, controller 190 is depicted schematically in FIG. 2 as a singlecontroller unit that is coupled to various components within furnace100. Particularly, controller 190 may comprise a processor 192 and amemory 194. Processor 192 (e.g., microprocessor, central processing unit(CPU), or collection of such processor devices, etc.) executesmachine-readable instructions 196 provided on memory 194 (e.g.,non-transitory machine-readable medium) to provide controller 190 withall the functionality described herein. Memory 194 may comprise volatilestorage (e.g., random access memory (RAM)), non-volatile storage (e.g.,flash storage, read-only memory (ROM), etc.), or combinations of bothvolatile and non-volatile storage. Data consumed or produced by themachine-readable instructions 196 can also be stored on memory 194. Asnoted above, in some embodiments, controller 190 may comprise acollection of controllers and/or control boards that are coupled to oneanother. As a result, in some embodiments, controller 190 may comprise aplurality of processors 192, memories 194, etc.

As described above, an HVAC system including an indoor unit and anoutdoor unit may include furnace 100 as a component of the indoor unitthereof. The HVAC system may include a system controller, which may bedisposed in a thermostat of the HVAC system and may be generallyconfigured to affect control over the indoor and outdoor units of theHVAC system. For example, the system controller may request a targetfiring rate of the burner assembly 140 of furnace 100 in response to anambient temperature of a comfort zone conditioned by the HVAC systemfalling below a user-defined set point temperature. In some embodiments,controller 190 may comprise a controller of furnace 100 that is separateand distinct from, but in selective communication with, one or morecontrollers or control boards of the HVAC system (e.g., the systemcontroller of the HVAC system, etc.). However, in other embodiments,controller 190 may comprise a plurality of controllers or control boardsof the HVAC system that are coupled to one another. For example, in someembodiments, controller 190 may comprise both a controller or controlboard of furnace 100 and the system controller of the HVAC systemdisposed in the thermostat thereof. Thus, in some embodiments, one ormore controllers or control boards of controller 190 may affect controlover components of the HVAC system other than furnace 100 (e.g., theoutdoor unit of the HVAC system, etc.).

In some embodiments, controller 190 may be configured to receiveinformation related to a speed and a torque of circulation fan 180whereby controller 190 may continuously determine the speed and thetorque of the motor 183 of circulation fan 180. Additionally, controller190 may be configured to estimate an airflow produced by circulation fan180 by monitoring one or more parameters of the motor 183 of circulationfan 180, such as a speed and a torque of motor 183. The one or moreparameters of motor 183 may be measured parameters of motor 183 and/orparameters determined from measured parameters of motor 183. Forexample, motor 183 may comprise one or more sensors configured tomeasure one or more parameters of motor 183, such as current, a counteror back electromotive force (EMF) of motor 183, a voltage supplied tomotor 183, etc. The measured parameters of motor 183 measured by the oneor more sensors thereof may be communicated to controller 190.Controller 190 may be configured to determine one or more parameters ofmotor 183, such as the speed and torque of motor 183, based on theparameters of motor 183 measured by the one or more sensors of motor 183and communicated to controller 190. Additionally, in some embodiments,controller 190 may be configured to determine one or more parameters ofthe motor 183 of circulation 180, such as a speed and a torque of motor183, required to achieve a desired or targeted airflow rate ofcirculation fan 180. For example, the controller 190 may monitor andadjust one or more measured parameters of motor 183 (e.g., a currentand/or voltage supplied to motor 183) to ensure a speed and torque ofmotor 183 required to achieve the targeted airflow rate is maintained.

Prior to installation of furnace 100, the furnace 100 (or another testfurnace, including a test circulation blower, similar in configurationto furnace 100) may be tested at an air plenum test facility at a rangeof known airflows (i.e., independently measured by equipment of the testfacility) to thereby create a motor map or discrete value look-up tablecorrelating airflow produced by circulation fan 180 with the motor speedand torque of motor 183 of circulation fan 180. As a non-limitingexample, a motor map may include airflow along an X-axis thereof, motorpower (which may be calculated from a determined motor torque) along aY-axis thereof, and a plurality of curves each corresponding to a fixedspeed of the motor of circulation fan 180. In this manner, an estimatedairflow may be “looked-up” from the motor map from a known motor speedand torque. However, additional functional relationships for airflow maybe used to correlate determined motor speed and torque with estimatedairflow.

Further, controller 190 may also be configured to determine a minimumairflow rate corresponding to an estimated rate of conditioned airflow184 of furnace 100 which corresponds to a maximum permissibleconditioned airflow temperature (e.g., maximum permissible conditionedairflow temperature of conditioned airflow 184) of furnace 100.Particularly, prior to installation of furnace 100, the furnace 100 (oranother test furnace similar in configuration to furnace 100) may betested at a test facility by activating the burner assembly 140 offurnace 100 and operating circulation fan 180 at a range of airflows(either independently measured by equipment of the test facility orestimated from the motor map). As the circulation fan 180 is operated ata range of airflows, the temperature of conditioned airflow 184 offurnace 100 may be independently measured by equipment of the testfacility to thereby estimate the airflow rate of conditioned airflow 184which corresponds to the maximum permissible conditioned airflowtemperature of furnace 100.

In some embodiments, the inlet airflow provided to the furnace (e.g.,furnace 100) during testing may be at a fixed nominal inlet airflowtemperature. The nominal inlet airflow temperature using during testingmay be a temperature that is near, at, or above a maximum inlet airflowtemperature the furnace is expected to receive during operationfollowing installation to ensure that the temperature of the inletairflow received by the furnace during operation does not substantiallyexceed the nominal airflow temperature used during testing.

In certain embodiments, during operation of the furnace, a controller(e.g., controller 190) may monitor a temperature of the inlet airflow(e.g., inlet airflow 182) and may compare the inlet airflow temperaturewith the nominal inlet airflow temperature used during testing andadjust the maximum permissible conditioned airflow temperature inresponse to the monitored inlet airflow temperature exceeding thenominal inlet airflow temperature. For example, the controller maydecrease the maximum permissible conditioned airflow temperature inproportion to the difference between the monitored inlet airflowtemperature and the nominal inlet airflow temperature (e.g., reduce themaximum permissible conditioned airflow temperature 5° F. in response tothe monitored inlet airflow temperature exceeding the nominal inletairflow temperature by 5° F., etc.). The temperature of the inletairflow received by the furnace may be determined using a dedicatedtemperature sensor positioned in a flowpath of the inlet airflow.Alternatively, the controller may monitor an indoor temperaturedetermined by, for example, a thermostat of an HVAC system comprisingthe furnace in order to estimate the temperature of the indoor airflowreceived by the thermostat.

In some embodiments, a conditioned airflow temperature map of furnace100 which correlates or maps the conditioned airflow temperature offurnace 100 (reflecting the temperature of the inlet airflow 182 as wellas the temperature rise of the airflow through the furnace 100) to theairflow produced by circulation fan 180 may also be produced duringtesting. For example, referring briefly to FIG. 3 , an exemplaryconditioned airflow temperature map 200 of a gas-fired furnace (e.g.,furnace 100) is shown. Conditioned airflow temperature map includes anairflow rate in cubic feet per minute (CFM) of the circulation fan 180of furnace 100 along an X-axis thereof (which may be estimated from adetermined speed and torque of the motor 183 of circulation fan 180utilizing a motor map), and conditioned airflow temperature (e.g.,temperature of conditioned airflow 184 of furnace 100) along a Y-axisthereof in degrees Fahrenheit (° F.). Additionally, conditioned airflowtemperature map 200 includes a curve 202 corresponding to the estimatedconditioned airflow temperature as a function of the airflow rateproduced by circulation fan 180. In this manner, an estimatedconditioned airflow temperature may be “looked-up” from conditionedairflow temperature map 200 from a determined speed and torque of themotor 183 of circulation fan 180. As shown in FIG. 3 , the airflow rateof circulation 180 is negatively correlated with the temperature ofconditioned airflow 184.

Referring again to FIGS. 1, 2 , the minimum airflow rate of furnace 100determined during testing may be stored in the memory of controller 190prior to the installation of furnace 100. In some embodiments, the motormap and the conditioned airflow temperature map created during testingmay also be stored in the memory of controller 190 prior to theinstallation of furnace 100.

In this manner described above, controller 190 of furnace 100 may applydetermined motor speed and torque values to the motor map andconditioned airflow temperature map stored in the memory of controller190 to thereby determine or look-up an estimated conditioned airflowtemperature of furnace 100 based on the determined motor speed andtorque of circulation fan 180. In some embodiments, the motor map andminimum airflow rate may be stored in the memory of controller 190, andcontroller 190 may apply determined motor speed and torque values to themotor map stored in the memory thereof to determine whether the airflowrate produced by circulation fan 180 falls below the minimum airflowrate of furnace 100.

In some embodiments, furnace 100 may not include a temperature limitswitch (e.g., a spring-operated bimetallic switch) for preventingfurnace 100 from exceeding a designed temperature rise range of furnace100. Instead, as further described below, controller 190 may beconfigured to prevent furnace 100 from exceeding the temperature riserange of furnace 100 without needing to rely on a separate temperaturelimit device or switch (e.g., a spring-operated bimetallic switch).

Furnace 100 may be operated to provide heat to one or more areas and/orcomfort zones of an indoor space by transferring heat from hotcombustion gases flowing along combustion flow path 172 generated byfurnace 100 to a conditioned airflow 184 that may be delivered to thecomfort zone of the indoor space. For example, controller 190 of furnace100 may “turn on” or activate the burner assembly 140 of furnace 100 byopening fuel supply valve 102 and operating igniter 144 and draftinducer 170 of furnace 100 to thereby combust fuel and air in burnerassembly 140 and/or heat exchanger 150 and induce a flow of combustiongases along combustion flow path 172. Additionally, as combustion gasesare circulated along combustion flow path 172, controller 190 mayoperate circulation fan 180 to receive an inlet airflow 182 andcirculate (e.g., blow or pull) air over the external surface 156 of heatexchanger 150. Circulation fan 180 may also be operated by controller190 to circulate the conditioned airflow 184 from furnace 100 to thecomfort zone of the indoor space. In some embodiments, controller 190may also cease activation or deactivate furnace 100 by “shutting off” ordeactivating the burner assembly 140 by closing the fuel supply valve102 and cease the operation of igniter 144 and draft inducer 170 tothereby cease the flow of combustion gases along combustion flow path172. Controller 190 may also cease the operation of circulation fan 180following the deactivation of burner assembly 140.

Referring now to FIG. 4 , a method 250 for operating a furnace is shown.In some embodiments, method 250 may be practiced with furnace 100. Thus,in describing the features of method 250, continuing reference will madeto the furnace 100 shown in FIGS. 1, 2 ; however, it should beappreciated that embodiments of method 250 may be practiced with othersystems, assemblies, and devices. Generally speaking, method 250includes monitoring one or more parameters indicative of an airflow rateof a conditioned airflow of a gas-fired furnace, and deactivating thefurnace in response to the one or more parameters indicating that theairflow rate of the conditioned airflow is less than a minimum airflowrate.

Initially, method 250 includes operating a gas-fired furnace (e.g.,furnace 100) to produce a conditioned airflow at method block 252. Theoperation of the furnace at block 252 may be performed at a testfacility prior to the installation of the furnace, or at an indoor space(e.g., a home, etc.) following the installation of the furnace. In someembodiments, method block 252 may include activating a burner assemblyand a first fan of the furnace (e.g., burner assembly 140 and draftinducer 170 of furnace 100) to combust fuel and air and circulatecombustion gases along a flow path (e.g., combustion flow path 172)extending through a heat exchanger of the furnace (e.g., heat exchanger150). For example, controller 190 may open fuel supply valve 102 andactivate burner assembly 140 and inducer fan 170 of furnace 100 tothereby combust fuel and air, which may be circulated through heatexchanger 150 along combustion flow path 172. Method block 252 may alsoinclude operating a second fan of the furnace (e.g., circulation fan180) to circulate air across the heat exchanger to produce a conditionedairflow. For example, controller 190 may operate circulation fan 180 tocirculate air over the external surface 156 of heat exchanger 150 andprovide a conditioned airflow 184 that may be delivered to a comfortzone of the indoor space.

Method 250 proceeds at method block 254 by monitoring one or moreparameters indicative of an airflow rate of the conditioned airflow. Insome embodiments, the one or more parameters may comprise one or moreparameters of a motor of the second fan (e.g., motor 183 of circulationfan 180). The one or more parameters of the motor may be determined fromone or more other parameters of the motor that are measured. Forexample, method block 254 may include determining a speed and a torqueof a motor of a circulation fan (e.g., motor 183 of circulation fan 180)of the furnace. In some embodiments, block 254 comprises determining aspeed and a torque of the motor 183 of the circulation fan 180 offurnace 100 as the circulation fan 180 provides the conditioned airflow184 exiting furnace 100. As described above, motor 183 of circulationfan 180 may communicate one or more measured parameters of motor 183 tocontroller 190, and controller 190 may be configured to determine thespeed and torque of motor 183 based on the measured parameters of motor183. The one or more measured parameters of motor 183 used to determinethe speed and torque of motor 183 may comprise a current and/or voltagesupplied to motor 183, a counter or back EMF of motor 183, etc. Methodblock 254 may include controller 190 continuously determining speed andtorque values of the motor 183 of circulation fan 180 during theoperation of furnace 100. In some embodiments, block 254 may includecontroller 190 determining other parameters of motor 183, such as ashaft or output power of motor 183.

Method block 254 may optionally include estimating an airflow rate ofthe conditioned airflow produced by the circulation fan (e.g.,circulation fan 180 of furnace 100) based on the determined speed andtorque of the motor of the circulation fan (e.g., motor 183 ofcirculation fan 180). For example, method block 254 may optionallyinclude estimating the airflow rate as circulation fan 180 of furnace100 produces the conditioned airflow 184. In this optional step, theairflow rate of the conditioned airflow produced by circulation fan 180may be estimated by controller 190 of furnace 100 as furnace 100 isoperated to produce the conditioned airflow 184. For example, controller190 may periodically estimate the airflow rate of circulation fan 180based on the determined speed and torque of motor 183 of circulation fan180 and a motor map stored in the memory of controller 190.

Method 250 continues at method block 256 by deactivating the burnerassembly of the furnace (e.g., burner assembly 140) in response to theone or more parameters indicating that the airflow rate is less than aminimum airflow rate. The minimum airflow rate may be predefined and maycomprise an airflow rate produced by the circulation fan thatcorresponds to a temperature of the conditioned airflow (e.g., thetemperature of conditioned airflow 184) equaling a maximum permissibleconditioned airflow temperature of the furnace. For example, asdescribed above, the minimum airflow rate may be determined from testingof the furnace at a test facility prior to installation where theairflow rate produced by the circulation fan is negatively correlatedwith the temperature of the conditioned airflow of the furnace. In someembodiments, the minimum airflow rate may be saved in the memory of acontroller (e.g., controller 190) prior to the installation of thefurnace.

The maximum permissible conditioned airflow temperature of the furnace(e.g., furnace 100) may comprise a temperature in excess of an upper endof a predefined temperature rise range of the furnace. Particularly, themaximum permissible conditioned airflow temperature of the furnace maycomprise a temperature above which damage due to overheating may resultto the furnace and/or heat-related discomfort may occur to occupants ofthe indoor area heated by the furnace. In some embodiments, the maximumpermissible conditioned airflow temperature of the furnace may comprisea temperature equal to an upper end of the designed temperature riserange of the furnace plus an additional fixed margin or safety factor.For instance, in an example where the fixed margin is equal to 100° F.and the designed temperature rise range of the furnace is between 30° F.and 60° F., the maximum permissible conditioned airflow temperature ofthe furnace may comprise 160° F. However, in other embodiments, themaximum permissible conditioned airflow temperature of the furnace mayvary.

In some embodiments, method block 256 comprises deactivating orshutting-off a burner assembly of the furnace whereby combustion of airand fuel in the furnace ceases. For example, controller 190 of furnace100, having the minimum airflow rate and a motor map of circulation fan180 stored in a memory thereof, may determine (based on the determinedspeed and torque of the motor 183 of circulation fan 180) that theairflow rate of conditioned airflow 184 produced by circulation fan 180is less than the minimum airflow rate. Controller 190 may close fuelsupply valve 102 and cease the operation of igniter 144 and draftinducer 170 to cease the gas flow through furnace 100 along combustionflow path 172 in response to determining that the airflow rate ofconditioned airflow 184 is less than the minimum airflow rate.Controller 190 may also deactivate or cease the operation of circulationfan 180 of furnace 100 following the deactivation of burner assembly 152to cease the production of conditioned airflow 184.

In some embodiments, a controller (e.g., controller 190 of furnace 100)may issue an alert to a user of the furnace (e.g., a homeowner, aninstaller of the furnace, and/or a technician equipped to service thefurnace) notifying the user that the burner assembly (e.g., burnerassembly 140) of the furnace has been deactivated in response to the oneor more parameters indicating that the airflow rate of the conditionedairflow produced by the circulation fan of the furnace (e.g.,circulation fan 180) has fallen below the minimum airflow rate of thefurnace so that the user may have the furnace serviced (e.g., replacinga filter of the furnace to reduce an obstruction to the circulation ofairflow through the furnace, etc.). In some embodiments, the controllermay place the furnace into an idle mode whereby operation of thefurnace, including the burner assembly and/or the circulation fanthereof, is prevented for a predetermined period of time to allow forthe burner assembly to cool before operation of furnace 100 may beresumed. Following the predetermined period of time, the controller maypermit the activation of the furnace, including the combustion of airand fuel in the furnace, to satisfy a demand for heating of the indoorspace.

In some embodiments, method 250 may optionally include operating a motorof the second fan at a speed or a torque that corresponds to a targetrate of the conditioned airflow. For example, controller 190 of furnace100 may operate the motor 183 of circulation fan 180 at a speed or atorque that corresponds to a target rate of conditioned airflow 184. Thetarget rate of conditioned airflow may correspond to a target firingrate of the burner assembly of the furnace called by a system controllerof an HVAC system comprising the furnace. For example, the systemcontroller may request a target firing rate in response to an ambienttemperature of a comfort zone conditioned by the HVAC system fallingbelow a user-defined set point of the HVAC system.

Method 250 may also include increasing the speed or the torque of themotor of the second fan in response to the one or more parametersindicating that the airflow rate produced by the second fan is less thanthe target airflow rate. Additionally, the burner assembly of thefurnace may be deactivated only in response to both the airflow ratebeing less than a predetermined minimum airflow rate, and either a speedor a torque of the motor of the second fan being at or above apredefined threshold. For example, controller 190 may continuouslyincrease the speed or the torque of the motor 183 of circulation fan 180in response to the one or more parameters indicating that the airflowrate of conditioned airflow 184 is less than the target rate untileither the speed or the torque of the motor 183 of circulation fan 180equals or exceeds a predefined threshold. The predefined threshold maycomprise a designed maximum speed or torque of the motor 183 ofcirculation fan 180. In response to the speed or the torque of the motor183 of circulation fan 180 being at or greater than the predefinedthreshold, and the airflow rate of circulation airflow 184 being lessthan the minimum airflow rate, controller 190 may deactivate burnerassembly 140 to cease the combustion of fuel and air in furnace 100.

In some embodiments, method 250 may further optionally includeestimating a temperature of the conditioned airflow of the furnace(e.g., the temperature of the conditioned airflow 184 of furnace 100)based on the estimated airflow rate of the conditioned airflow (e.g.,conditioned airflow 184). For example, a controller (e.g., controller190 of furnace 100) may periodically determine or estimate thetemperature of the conditioned airflow of the furnace based on aconditioned airflow temperature map of the furnace stored in the memoryof the controller. As described above, the conditioned airflowtemperature map may be created during testing of the furnace at atesting facility prior to installation. In some embodiments, theconditioned airflow temperature map may be pre-stored in the memory ofthe controller prior to installation of the furnace. Additionally, insome embodiments, the minimum airflow rate may be determined from theconditioned airflow temperature map of the furnace, where the minimumairflow rate corresponds to the point along the curve of the conditionedairflow temperature map where the estimated conditioned airflowtemperature equals the maximum permissible conditioned airflowtemperature of the furnace.

Referring now to FIG. 5 , another method 270 for operating a furnace isshown in FIG. 5 . In some embodiments, method 270 may be practiced withfurnace 100 shown in FIGS. 1, 2 . Thus, in describing the features ofmethod 270, continuing reference will made to the furnace 100 shown inFIGS. 1, 2 ; however, it should be appreciated that embodiments ofmethod 270 may be practiced with other systems, assemblies, and devices.Generally speaking, method 270 includes monitoring a parameter that isindirectly indicative of a temperature of a conditioned airflow producedby a gas-fired furnace, and deactivating the furnace in response to theparameter indirectly indicating that the temperature of the conditionedairflow exceeds a threshold.

Initially, method 270 includes operating a gas-fired furnace (e.g.,furnace 100) to produce a conditioned airflow at method block 272.Method block 272 may be similar to the method block 252 of method 250described above. For instance, method block 272 may include activating aburner assembly and a first fan of the furnace (e.g., burner assembly140 and draft inducer 170) to combust fuel and air and circulatecombustion gases along a flow path (e.g., combustion flow path 172)extending through a heat exchanger of the furnace (e.g., heat exchanger150), and operating a second fan of the furnace (e.g., circulation fan180) to circulate air across the heat exchanger to produce a conditionedairflow.

Method 270 proceeds at method block 274 by monitoring a parameter thatis indirectly indicative of a temperature of the conditioned airflow. Asused herein, “indirectivity indicative” refers to a relationship betweentwo variables that is not directly proportional such that the twovariables do not correspondingly increase or correspondingly decrease inthe same ratio. In other words, the monitored parameter is not directlyproportional to the temperature of the conditioned airflow. Theparameter may be negatively correlated with the temperature of theconditioned airflow. In some embodiments, the parameter may comprise orbe indicative of an airflow rate of the conditioned airflow or a speedand a torque of a motor of a second fan of the furnace (e.g., motor 183of the circulation fan 180 of furnace 100). For example, controller 190of furnace 100 may monitor speed and torque of the motor 183 ofcirculation fan 180 and estimate the airflow rate of circulation airflow184 based on the monitored speed and torque of motor 183 as well as amotor map and a conditioned airflow temperature map stored in the memoryof controller 190. As described above, the airflow rate of theconditioned airflow is negatively correlated with, and thus may beindirectly indicative of, the temperature of the conditioned airflow.

Method 270 continues at method block 276 by deactivating a burnerassembly of the furnace (e.g., burner assembly 140 of furnace 100) inresponse to the parameter indirectly indicating that the temperature ofthe conditioned airflow exceeds a threshold. In some embodiments, thethreshold may comprise a maximum permissible conditioned airflowtemperature of the furnace. Thus, method block 276 may comprisedeactivating the burner assembly of the furnace in response to theparameter indirectly indicating that the temperature of the conditionedairflow exceeds the maximum permissible conditioned airflow temperatureof the furnace. For example, controller 190 of furnace 100 maydeactivate burner assembly 140 to cease the combustion of fuel and airin furnace 100 in response to the parameter indirectly indicating thatthe temperature of conditioned airflow 184 exceeds the maximumpermissible conditioned airflow temperature of the furnace. As describedabove with respect to method block 274, the parameter may comprise, orbe indicative of, an airflow rate of the conditioned airflow or a speedand a torque of a motor of a second fan of the furnace (e.g., motor 183of the circulation fan 180 of furnace 100).

Referring to FIGS. 4, 5 , through use of the systems and methodsdescribed herein (e.g., furnace 100, methods 250, 270, etc.), a furnacemay be deactivated in response to a conditioned airflow produced by acirculation fan of the furnace being less than a minimum airflow raterequired to maintain a temperature of the conditioned airflow equal toor less than a maximum permissible conditioned airflow temperature ofthe furnace.

Specifically, a gas-fired furnace (e.g., furnace 100 shown in FIGS. 1, 2) may be operated to produce a conditioned airflow (e.g., producing aconditioned airflow at method blocks 252, 272 of methods 250, 270,respectively), monitoring one or more parameters indicative of anairflow rate of the conditioned airflow (e.g., monitoring one or moreparameters indicative of the airflow rate at block 254 of method 250)and deactivating a burner assembly of the furnace in response to the oneor more parameters indicating that the airflow rate of the conditionedairflow is less than the minimum airflow rate (e.g., deactivating aburner assembly of the furnace at block 256 of method 250).Additionally, a gas-fired furnace may be operated by monitoring aparameter that is indirectly indicative of a temperature of theconditioned airflow (e.g., monitoring the parameter at method block 274of method 270), and deactivating the burner assembly of the furnace inresponse to the parameter indirectly indicating that the temperature ofthe conditioned airflow exceeds a threshold (e.g., deactivating thefurnace at block 274 of method 270).

In this manner, the furnace may be deactivated once the temperature ofthe conditioned airflow produced by the furnace exceeds the maximumpermissible conditioned airflow temperature of the furnace withoutneeding to rely on a separate temperature limit switch (e.g., aspring-operated bimetallic switch). As described above, the temperaturelimit switch rendered superfluous by embodiments disclosed herein mayadd to the overall expense of the furnace while also requiring thefurnace to be configured and/or installed in a particular orientation(limiting the flexibility in which the furnace may be internallyconfigured and/or installed in an indoor space) in order to function asintended. Although the elimination of the temperature switch using themethods described above (e.g., methods 250, 270) is discussed in thecontext of gas-fired furnaces, methods described herein may be appliedto prevent other heating units of HVAC systems, such as electricallypowered supplemental or auxiliary heaters, from exceeding a maximumpermissible conditioned airflow temperature.

While exemplary embodiments have been shown and described, modificationsthereof can be made by one skilled in the art without departing from thescope or teachings herein. The embodiments described herein areexemplary only and are not limiting. Many variations and modificationsof the systems, apparatus, and processes described herein are possibleand are within the scope of the disclosure. Accordingly, the scope ofprotection is not limited to the embodiments described herein, but isonly limited by the claims that follow, the scope of which shall includeall equivalents of the subject matter of the claims. Unless expresslystated otherwise, the steps in a method claim may be performed in anyorder. The recitation of identifiers such as (a), (b), (c) or (1), (2),(3) before steps in a method claim are not intended to and do notspecify a particular order to the steps, but rather are used to simplifysubsequent reference to such steps.

What is claimed is:
 1. A method for operating a furnace, comprising: (a)activating a burner assembly and a first fan of the furnace to combustfuel and air and circulate combustion gases along a flow path extendingthrough a heat exchanger of the furnace; (b) operating a second fan ofthe furnace to circulate air across an external surface of the heatexchanger of the furnace and produce a conditioned airflow; (c)estimating an airflow rate of the conditioned airflow based on two ormore parameters of a motor of the second fan, wherein at least one ofthe two or more parameters is indicative of a speed of the motor of thesecond fan and at least another of the two or more parameters isindicative of a torque of the motor of the second fan; (d) deactivatingthe burner assembly, whereby combustion of the fuel and air in thefurnace ceases, in response to an indication by a combination of thespeed and torque parameters that the estimated airflow rate is less thana minimum airflow rate; and (e) placing the furnace in an idle mode,wherein the idle mode prevents the burner assembly from operating for apredetermined period of time.
 2. The method of claim 1, furthercomprising: issuing an alert to a user of the furnace in response to theindication that the estimated airflow rate is less than the minimumairflow rate.
 3. The method of claim 1, wherein the minimum airflow rateis predefined.
 4. The method of claim 1, further comprising: (g)operating the motor of the second fan at a speed and a torque thatcorresponds to a target rate of the conditioned airflow and a targettemperature rise range; and (h) increasing the torque of the motor ofthe second fan in response to the two or more parameters indicating thatthe airflow rate is less than the target rate.
 5. The method of claim 4,wherein (d) comprises deactivating the burner assembly in response tothe speed or the torque of the motor of the second fan being at or abovea predefined threshold.
 6. The method of claim 1, wherein the at leastone parameter indicative of the speed of the motor of the second fanincludes current, and wherein the at least another parameter indicativeof the torque of the motor of the second fan includes one or more of acounter electromotive force, a back electromotive force, or a voltage.7. The method of claim 4, wherein the target temperature rise range isbetween 30 degrees Fahrenheit (° F.) and 60° F.
 8. A gas-fired furnace,comprising: a burner assembly configured to combust fuel and air toproduce combustion gases; a first fan configured to circulate thecombustion gases along a flow path extending through a heat exchanger ofthe furnace; a second fan configured to circulate air across an externalsurface of the heat exchanger to produce a conditioned airflow; and acontroller configured to: activate the burner assembly and the first fanto combust the fuel and air and circulate the combustion gases along theflow path; operate the second fan to circulate the air across theexternal surface of the heat exchanger and produce the conditionedairflow; estimate an airflow rate of the conditioned airflow based ontwo or more parameters of a motor of the second fan, wherein at leastone of the two or more parameters is indicative of a speed of the motorof the second fan and at least another of the two or more parameters isindicative of a torque of the motor of the second fan; deactivate theburner assembly whereby combustion of the fuel and air ceases inresponse to an indication by a combination of the speed and torqueparameters that the estimated airflow rate is less than a minimumairflow rate; and place the furnace in an idle mode, wherein the idlemode prevents the burner assembly from operating for a predeterminedperiod of time.
 9. The furnace of claim 8, wherein the controller isconfigured to issue an alert to a user of the furnace in response to theindication that the estimated airflow rate is less than the minimumairflow rate.
 10. The furnace of claim 8, wherein the minimum airflowrate is predefined.
 11. The furnace of claim 8, wherein the controlleris configured to: operate the motor of the second fan at a speed and atorque that corresponds to a target rate of the conditioned airflow anda target temperature rise range; and increase the torque of the motor ofthe second fan in response to the two or more parameters indicating thatthe airflow rate is less than the target rate.
 12. The furnace of claim11, wherein the controller is configured to deactivate the burnerassembly in response to the speed or the torque of the motor of thesecond fan being at or above a predefined threshold.
 13. The furnace ofclaim 8, wherein the at least one parameter indicative of the speed ofthe motor of the second fan includes current, and wherein the at leastanother parameter indicative of the torque of the motor of the secondfan includes one or more of a counter electromotive force, a backelectromotive force, or a voltage.
 14. The furnace of claim 11, whereinthe target temperature rise range is between 30 degrees Fahrenheit (°F.) and 60° F.
 15. A non-transitory machine-readable medium includinginstructions that, when executed by a processor, cause the processor to:activate a burner assembly and a first fan of a furnace to combust fueland air and circulate combustion gases along a flow path extendingthrough a heat exchanger of the furnace; operate a second fan of thefurnace to circulate air across an external surface of the heatexchanger of the furnace and produce a conditioned airflow; estimate anairflow rate of the conditioned airflow based on two or more parametersof a motor of the second fan, wherein at least one of the two or moreparameters is indicative of a speed of the motor of the second fan andat least another of the two or more parameters is indicative of a torqueof the motor of the second fan; deactivate the burner assembly, wherebycombustion of the fuel and air ceases, in response to an indication by acombination of the speed and torque parameters that the estimatedairflow rate is less than a minimum airflow rate; place the furnace inan idle mode, wherein the idle mode prevents the burner assembly fromoperating for a predetermined period of time.
 16. The non-transitorymachine-readable medium of claim 15, wherein the instructions, whenexecuted by the processor, further cause the processor to issue an alertto a user of the furnace in response to the indication that theestimated airflow rate is less than the minimum airflow rate.
 17. Thenon-transitory machine-readable medium of claim 15, wherein theinstructions, when executed by the processor, further cause theprocessor to: operate the motor of the second fan at a speed and atorque that corresponds to a target rate of the conditioned airflow anda target temperature rise range; and increase the torque of motor of thesecond fan in response to the two or more parameters indicating that theairflow rate is less than the target rate.
 18. The non-transitorymachine-readable medium of claim 17, wherein the instructions, whenexecuted by the processor, further cause the processor to deactivate theburner assembly in response to the speed or the torque of the motor ofthe second fan being at or above a predefined threshold.
 19. Thenon-transitory machine-readable medium of claim 15, wherein the at leastone parameter indicative of the speed of the motor of the second fanincludes current, and wherein the at least another parameter indicativeof the torque of the motor of the second fan includes one or more of acounter electromotive force, a back electromotive force, or a voltage.20. The non-transitory machine-readable medium of claim 17, wherein thetarget temperature rise range is between 30 degrees Fahrenheit (° F.)and 60° F.